A ram pump, a water powered water pump, can be used to compress air by introducing it
below the delivery valve. The air rises inside the pump and when water is pushed through the
delivery valve, the air is forced along with it.

The pressure limit for the air is about the same as for water pumping - about 180 metres (18
bars = 260 p.s.i.). There are all sorts of limitations on such a system so don't jump to
conclusions, but the point is that it is possible and it is a form of renewable energy that is
water-powered for nearly no cost once you have a ram pump of this construction.

All ram pumps using a hydraulic accumulator with pressurized air (not all do) already
compress air but in small quantities. This is used to maintain the air level above the delivery
valve. The air gets absorbed when the water pressure rises and the air-poor water entering the
chamber takes air from the accumulator. The snifter valve, which is tiny and has a very small
orifice, allows a little air to enter the pump body on each stroke. Making this into an air
compressor involved fitting a larger tuneable valve.

The amount of air you can get compressed is related to the power available from the falling
water. There is nothing for free so more power equals more compressed air. There must
always be a little water (at least) pumped with the air to maintain the water seal on the delivery
valve (the inverse of normal pumping where the air maintains the accumulator). Just as you
always get a little air in the pipes with a regular ram pump, you get some water in the
compressed air.

Disadvantages:

The air is wet and some applications require dry air. Drying will have to be done separately in
such cases. The water (which is under pressure) can be vented, collected or sent up a pipe - a
different one if you like.

Advantages:

The water cools the air as it is compressed and raises the efficiency of the operation. You get
more compressed air per KW of water energy that you would get from an electric motor
driven piston compressor because the compressing does not heat the air as much when it is
pressurized inside a ram pump. The temperature is kept low by the water surrounding the air.
There is a fancy engineering name for this type of compression.

Compressed air can be transported up hills without head losses (which happens with water)
and it can be stored both in the pipelines and in any number of tanks attached around the
system.

There are numerous power tools which will run on air power, as typically everything
underground in a mine is air-powered, not electrically driven. Mechanical work can be done
without first converting the air to electricity and then to mechanical work.

Small amounts of electricity can be generated using compressed air but it is notoriously
inefficient. It would, however, be able to charge a cell phone or run a radio as an added
feature of air-powered system.

There is one model of compact car that runs on compressed air. It was designed by a Formula
One mechanic and is made in France. You can take large volumes of compressed air and,
using air, turn it into a smaller amount of very high pressure air to 'fuel' the car, basically for
free.